Rato, also a former Spanish economy minister, was taken in an unmarked police car accompanied by officers. Spanish media reported he was heading with them to one of his offices and would later go to a police station.

Television images showed Rato leaving the apartment building accompanied by an agent who put his hand on top of Rato's head as he entered the car to make sure he didn't bump it. Rato was dressed in a suit without a tie and was not handcuffed.

Madrid's regional court said in a statement that a judge granted a request from tax authorities to enter and search Rato's house as part of a legal case that officials are building. It provided no further details, saying the investigation is secret.

Rato headed the IMF from 2004 to 2007. Before that, he was a prominent member of Spain's Popular Party, which currently governs the country but has had its reputation severely damaged by a series of corruption scandals.

Rato is also under investigation for his former role as chief executive of Spanish bank Bankia, which had to be bailed out in 2012.